Induced radiative effects such as Raman scattering and fluorescence have become extremely valuable investigative tools. To characterize a composition in a remote or hostile environment, optical fibers may advantageously be used to deliver excitation energy to a sample under investigation and to carry scattered radiation back to means for spectral analysis. An excitation source path may take the form of a laser providing a stimulus at an appropriate wavelength coupled to an input fiber, and a collection path may be made up of a second fiber carrying return radiative information to a spectral analysis tool such as a spectrograph.
Such remote spectral analysis presents technical challenges, however, including the strong scattering signature of the material used for the optical fiber, this interference potentially being generated by both the laser excitation in the illumination fiber and any strong Rayleigh (unshifted) Scattering allowed to enter the collection fiber. These spurious fiber signatures can compete with, or even overshadow, the desired signature of the sample under test, particularly when long lengths of fiber are used.
In a typical arrangement, energy from an excitation laser is coupled into the illumination fiber. This stimulation begins as a relatively pure, single wavelength of light, but while traveling through the fiber, the laser energy induces Raman scattering within the fiber material, typically composed of silica, yielding a spectrum at the output of the illumination fiber which contains spurious Raman lines in addition to the laser wavelength.
Unless these undesired lines are eliminated from the illumination path before reaching the sample, their Rayleigh scatter at the sample may be indistinguishable from the true, shifted Raman scatter due to the laser excitation of the sample. Therefore, a laser band pass device is used to remove these unwanted wavelengths, thereby outputting, ideally, the single laser line to the illumination optic and sample under characterization. This assumes, of course, that the illumination optic contains a sufficiently short optical path that it does not itself generate significant spurious scattering.
The light scattered by the sample is collected by a collection optic, which may be the same element as the illumination optic used in counter-propagating fashion. At the output of the collection optic, the scattered radiation consists of the unshifted Rayleigh scatter at the laser wavelength and the shifted Raman scatter that characterizes the sample under test. Since the Rayleigh scatter is several orders of magnitude stronger than the Raman scatter, if allowed to enter collection fiber, this strong Rayleigh scatter can excite spurious Raman scattering within the collection fiber similar to this situation within illumination fiber.
This Rayleigh scatter must therefore be rejected before being coupled to collection fiber. This may be accomplished with a Rayleigh rejection element to remove the strong Rayleigh line. The collection fiber then conducts only the relatively weak Raman scattering lines from the sample to an analysis instrument such as a spectrograph for detection. Particularly in more modern instruments, holographic notch filters are used as narrowband reflective elements to reject the Rayleigh scatter.